1. Field of the Invention
The present invention generally relates to a communications system which utilizes radio communications, and more particularly, to positioning method and apparatus for detecting the position of a mobile station such as a portable telephone.
2. Description of Related Art
In recent years, communications systems utilizing radio communications have been rapidly popularizing, and further progress is expected, for example, in such fields as mobile communications systems such as portable telephones and pagers, navigation systems for detecting the position of a moving person or vehicle, and the like.
For promoting the further progress of such communications systems, the development is desired for a positioning apparatus which is capable of accurately determining the position of a mobile station such as a movable portable telephone, navigation apparatus and the like, establishing a stable communication state between a base station and the mobile station based on the positioning result, and supporting widely diversified mobile communications systems and navigation systems.
As a positioning method for use in a conventional positioning apparatus, a positioning system in a mobile communications system in accordance with a code division multiple access scheme (hereinafter simply called the “CDMA scheme”) is known.
In this positioning method, as illustrated in FIG. 1, a mobile station P, which is a mobile radio terminal such as a portable telephone, receives radio waves transmitted from a plurality of base stations, for example, A, B, C installed in a communication area of the communications system. The propagation ranges of the radio waves are calculated from propagation times of the respective radio waves taken to arrive at the mobile station P from the respective base stations A, B, C. Then, the position of the mobile station P is detected by an analysis, to which the triangulation is applied, based on known position information of the respective base stations.
Describing more specifically a procedure of the conventional positioning system, the mobile station P is provided with a positioning apparatus which comprises a receiving part 1, and a range measureing part 6 and a position calculating part 7 connected to the receiving part 1, as illustrated in FIG. 2.
Specifically, in the mobile station P which comprises the receiving part 1, transmitting part 2, high frequency signal processing part (RF part) 3 and transmission/reception antenna ANT for performing radio communications with a base station, as the antenna ANT receives incoming radio waves from the respective base stations A, B, C, the high frequency signal processing part 3 converts the frequency of the received signal recovered from the radio waves. Subsequently, the downconverted signal is converted to digital data Dd which is further passed through a roll off filter 4, and despread in a demodulator 5 to generate received data Drx. Then, the position measureing part 6 and position calculating part 7 provided in the mobile station P performs the aforementioned triangulation-based analysis using Dd which is the output of the roll off filter 4, and Drx which is the output of the demodulator 5, to detect the current position of the mobile station P.
The range measureing part 6 illustrated in FIG. 2 is provided with a correlator 8 and a range calculating part 9, as illustrated in FIG. 3. The correlator 8 calculates correlation values between correlation data DA, DB, DC correlated to incoming radio waves from the respective base stations A, B, C, and Dd which the output of the roll off filter 4, respectively. The range calculating part 9 in turn analyzes the correlation values CRRA, CRRB, CRRC calculated by the correlation calculation to derive propagation ranges LA, LB, LC of the respective incoming radio waves.
Specifically, as illustrated in FIGS. 4A through 4C, as the correlator 8 calculates correlation values CRRA, CRRB, CRRC corresponding to the incoming radio waves from the respective base stations A, B, C, the range calculating part 9 compares these correlation values with a predetermined threshold value THC to detect a peak value of each correlation value. Subsequently, the range calculating part 9 calculates delay times tA, tB, tC to the detection of the respective peak values. Then, regarding these delay times as propagation times of the radio waves arriving from the respective base stations, the range calculating part 9 converts the delay times to propagation ranges to derive the propagation ranges LA, LB, LC of the respective incoming radio waves.
The position calculating part 7 performs the aforementioned triangulation-based analysis using the propagation ranges LA, LB, LC to find the current position of the mobile station P. Specifically, each of the base stations A, B, C is to transmit position information of each base station (the latitude and longitude at which each base station exists) on a transmitted radio wave. Therefore, in a communication between the mobile station P and each base station, the position calculating part 7 extracts the position information of each base station from received data Drx, and performs the triangulation-based analysis using the position information and the aforementioned propagation ranges LA, LB, LC to find position data Dp indicative of the current position of the mobile station P.
However, the foregoing conventional positioning method has a problem in that it is affected by so-called multipath fading and noise to degrade the positioning accuracy, and that it encounters difficulties in improving the positioning accuracy due to its susceptibility to such external factors.
For giving a specific example in explaining this problem, reference is made to FIG. 5. Specifically, suppose that an obstacle BL1 such as a building exists between the base station A and the mobile station P, causing a reduction in the level of direct wave emitted from the base station A to the mobile station P. Suppose further that the direct wave from the base station A is reflected by reflecting objects BL2, BL3 such as buildings, so that they arrive at the mobile station P as so-called multipath waves.
In this case, as illustrated in FIG. 6A, a plurality of peaks appear due to the direct wave and multipath waves in a correlation value CRRA which is the output of the correlator 8 in the range measureing part 6 of the mobile station P. Then, if the plurality of peak values appear as larger values than the predetermined threshold value THD, it is impossible to determine which peak is attributable to the direct wave. For this reason, the conventional positioning method has a problem in that it could erroneously determine a peak of the correlation value caused by a multipath wave as a peak attributable to the direct wave.
Also, due to the influence of the obstacle BL1, the level of the direct wave arriving at the mobile station P becomes relatively lower, as compared with the level of the multipath waves, so that the peak value attributable to the direct wave is lower than the threshold value THD, and the peak value attributable to the multipath wave exceeding the threshold value THD. In this case, a problem arises that the range calculating part 9 in the range measureing part 6 could determine that a delay tAe to the appearance of the peak due to the multipath wave is attributable to the direct wave, as illustrated in FIG. 6B.
Further, the situation as illustrated in FIGS. 6A, 6B can be encountered, in addition to the influence of the multipath waves, when the mobile station receives noise correlated to correlation data DA corresponding to the base station A, and a peak appears in a correlation value CRRA due to the nose, causing difficulties in distinguishing the direct wave from such noise.
When the propagation range LAe calculated from the position tAe at which a peak of a correlation value appears due to the multipath waves or noise is erroneously determined as the range from the base station A, a position Pe deviated from the essential position (true position) of the mobile station P is determined as the current position of the mobile station, as illustrated in FIG. 7, resulting in a degraded positioning accuracy.
While the foregoing exemplary case has been described for the case where a direct wave from the base station A cannot be accurately detected, it is possible in an actual operation that a direct wave cannot be accurately detected due to disturbance such as multipath waves for the remaining base stations B, C. This makes an improvement in positioning accuracy more difficult.
Specifically, the positions of the base stations A, B, C are known, so that if the propagation ranges LA, LB, LC of direct waves from the respective base stations can be accurately detected, the true position of the base station P can be determined by drawing three circles centered at the centers of the respective base stations A, B, C and having radii equal to the propagation ranges LA, LB, LC of direct waves from the respective base stations, using the triangulation, and finding a point at which the three circles intersect. However, under an actual communication environment in which detected range values from the base stations A, B, C to the mobile station P include randomly varying errors due to the influence of multipath fading and noise, the conventional positioning method could erroneously recognize a variety of positions within a hatched region shown in FIG. 7 as the current position of the mobile station